1. Field of the Invention
The present invention is directed to a device and method for preventing settlement of particles on a chemical-mechanical polishing pad, and more particularly, to a device placed between the polishing pad and a platen of a chemical-mechanical polishing apparatus used during the fabrication of semiconductor devices.
2. Discussion of the Prior Art
During fabrication of semiconductor devices, irregular top surfaces result due to many manufacturing processes, such as forming conductive lines and single or multiple layers. During various stages of semiconductor wafer production, irregular top surfaces of the wafers are planarized or flattened to provide smooth surfaces. Planarized surfaces improve performance and yield of integrated circuits formed on the wafer.
Chemical-mechanical polishing (CMP) is one method to planarized wafer surfaces. In the CMP process, the wafer is rubbed with a polishing pad. The rubbing is accomplished by pressing the wafer or polishing pad toward each other, and rotating one or both of them relative to each other. A slurry is used to chemically/mechanically attack the wafer surface and facilitate removal thereof by the mechanical abrasion provided by the rotating polishing pad.
Particles are generated from wafer abrasion (i.e., the mechanical abrasion of the wafer surface being polished), from slurry agglomeration (i.e., slurry particles that coalesce, which slurry particles are approximately 0.05 microns in size), and from pad debris resulting from polishing pad disintegration. These particles embed or settle within the polishing pad fabric, and protrude during the polish process causing wafer scratching, defects and improper planarazation.
Furthermore, the embedded particles change the surface structure of the polishing pad, resulting in process instability, and reduced repeatability and polish or removal rate. In extreme cases, the decline in the polish or removal rate results in an incomplete removal of material, leading to degradation in polishing uniformity. Polishing uniformity is further degraded due to particles being embedded on the pad in a non-uniform fashion. For example, a particular area of the pad may have more particles embedded therein than other areas. This non-uniformity is further accentuated when the wafer surface contains areas of different material, that are removed or polished at different rates.
The embedded particles also reduce the pad useful life, thus requiring frequent changing of the polishing pads. In addition to the cost of the pads, replacing the pads interrupts the wafer manufacturing process and reduces efficiency and yield. Moreover, this necessitates conditioning the pads prior to use, e.g., by planarizing the pads prior to use.
To prevent particles from being embedded in the polishing pad, an ultrasonic transducer has been placed in the slurry to vibrate or agitate the slurry. The ultrasonic transducer is either suspended in the slurry, or rests on the polishing pad. Alternatively, the ultrasonic transducer has been placed in contact with the wafer being polished, or placed under a platen disk onto which the polishing pad is attached, on a side of the platen that is opposite the pad polishing side. However, conventional devices using an ultrasonic transducer do not provide flexibility in providing directed vibration to specifically desired regions of the pad. This causes regional defects on the polished wafer surface and requires frequent changing of the polishing pad.
In addition to the slurry vibration, a large portion of the CMP device is also vibrated, causing undue wear and noise that degrade performance of the CMP device. This results in slow polishing rates and defective wafer polishing. Accordingly, there is a need for a versatile CMP device that variably controls prevention of particles from being embedding in the polishing pad at specific desired locations, and reduces vibration of portions of the CMP device that are adversely affected by undesired vibration.